Electrosurgical apparatus has associated with it an electric shock harzard. This is of particular concern with an electrosurgical generator where electrodes are connected directly to a patient.
With a conventional electrosurgical generator, a plate electrode is connected to earth and, therefore, connects the body of the patient directly to earth. One of the most common causes of electric shock is the passage of an electric current from a "live" source in contact with the patient to earth via the plate electrode. The "live" source may be an electrode from another piece of equipment which has become live due to a fault within the equipment.
This electric shock hazard may be prevented by isolating an output circuit of the electrosurgical generator from earth to provide a very high impedance to earth to the passage of current at the power supply frequency. However, this form of isolated output circuit itself produces a hazard in that the output of the electrosurgical generator can appear between the plate circuit and earth. This could cause burns to the patient or to any person in contact with the plate circuit when the generator is energised. The condition will occur if a low impedance path exists between the active output terminal and earth, for example if the active electrode is placed upon earthed metal.
It is well known that for safety use of an electrosurgical apparatus it is essential to ensure that the plate electrode is connected to the generator and that there is continuity throughout the connecting lead. Hitherto, it has been normal practice to include in an electrosurgical generator a monitor circuit that passes a small current through the plate electrode circuit, with a relay or similar means to detect the passage of this current if the circuit is complete. The relay can be arranged to operate an alarm if there is a loss of continuity in the plate circuit. This system has certain disadvantages, namely: (a) a twin conductor plate electrode lead is required, (b) the monitoring current can produce an electric shock hazard, (c) the monitoring current can cause interference with patient monitoring apparatus, and (d) the monitor circuit requires additional circuit components connected to the plate electrode circuit increasing the difficulty of maintaining satisfactory electrical isolation.
An object of the present invention is to provide a way of making an electrosurgical generator with an isolated output circuit, but without the hazard of diathermy burns from the plate circuit.
A further object of the invention is to eliminate the need for a plate electrode lead continuity monitor by providing an arrangement in which if the plate electrode is not connected or the lead is broken and an attempt is made to use the apparatus, the output voltage on the plate circuit will rise above the trigger level of the monitor, causing the generator to be switched off. To prevent the voltage exceeding the trigger level the plate must be connected to provide a lower capacitive impedance to earth.